Logarithmic amplifiers are used in many applications where signals of large dynamic range may be encountered. A logarithmic amplifier effectively compresses the dynamic range of the input signals, producing output signals whose magnitudes are logarithmically related to the magnitudes of the input signals. The compressed output signals may then be applied to signal processing circuitry having a dynamic range less than that associated with the input signals.
One technique used to produce a logarithmic output signal is known as successive detection. In this approach a series of cascaded amplifiers are connected such that an input signal applied to a first amplifier is amplified and applied to a second amplifier in the series. The second amplifier amplifies the signal and applies it to a third amplifier in the series, and so on. The output of each amplifier is also directed to a corresponding detector through, for example, an RF coupler or power splitter. When the RF signal energy produced by a particular amplifier exceeds a detector threshold, the detector coupled to the amplifier produces a detection signal corresponding to the video envelope of the detected RF energy. As the magnitude of the input signal is increased, the cascaded amplifiers successively saturate. Specifically, the final amplifier in the cascade saturates first, followed by the next to last amplifier, and so on. Since when an amplifier saturates it is producing a maximum RF output, the magnitude of the detection signal produced by the associated detector is also at a maximum.
The detectors are designed so that when an amplifier produces RF signal energy that saturates the succeeding amplifier, the signal energy also exceeds the corresponding detector threshold. Hence, for a given input signal the detectors associated with each saturated stage are producing maximum detection signals, while the amplifier preceding the sequence of saturated stages is operative in the linear regime. The composite video output signal corresponding to this input signal is then formed by summing the detection signals produced by each stage. The composite video output signal provides a piecewise linear approximation to an ideal logarithmic response to the input signal, wherein the accuracy of the piecewise approximation is inversely proportional to the gain of the RF amplifier stages.
As is indicated above, for input signals within the input dynamic range of a successive detection logarithmic amplifier a number of the constituent RF amplifiers will be operative in a saturation mode. When the magnitude of the applied input suddenly decreases (e.g., at the conclusion of a high energy pulse), there ensues a recovery period during which the transistors and associated bias circuitry within the saturated amplifier stages transition from saturation to small signal operation. During this recovery period the amplifier exhibits decreased ability to accurately respond to low level pulses arriving shortly after the high level signal responsible for inducing saturation.
Conventional implementations of the detectors associated with the cascaded RF amplifiers also tend to compromise response capability. For example, at microwave frequencies the response time, i.e., recovery time, of the logarithmic amplifier to step decreases in RF input signal intensity is degraded by the slow turn-off of biased Schottky detector circuits. As a consequence, RF couplers in conjunction with tunnel diodes operative at lower RF input levels have been utilized for RF detection. Unfortunately, realization of detectors involving this combination of elements tends to be relatively expensive, and is often impractical in many applications given the large size of RF couplers.
It is also possible to tap the output of the final RF amplifier within the cascade so as to obtain a nearly constant-power RF output, generally referred to as an RF "limited" output. The RF-limited outputs of conventional successive-detection logarithmic amplifiers have tended to be of relatively poor quality, typically exhibiting high levels of undesired harmonics and a considerable dependence in phase on the magnitude of the RF input. In contrast, an ideal limited RF output would be of a fixed magnitude for input levels above a specific threshold. Moreover, such an ideal RF output would be of a phase, although delayed in time, substantially identical to the input signal without being dependent upon the magnitude thereof. Finally, it is desired that RF limited outputs be relatively free of even harmonics so as to improve signal to noise ratio.